1. A composition comprising a conjugate of an antibody exhibiting binding specificity for an extracellular epitope of c-erbB-2 protein and a plant derived toxin, wherein saidtoxin is pharmacologically effective against neoplastic cells and is selected from the group consisting of gelonin, full length recombinant gelonin and functional gelonin fragments, wherein said conjugate is a fusion protein between said antibody andsaid toxin.

2. The composition of claim 1, wherein said antibody is an intact full-length immunoglobulin.

3. The composition of claim 1, wherein said antibody is an Fv fragment, and the V.sub.H peptide is conjugated with said toxin.

4. The composition of claim 1, wherein said antibody is an sFv, and the V.sub.L peptide is conjugated with said toxin.

5. The composition of claim 1, wherein said antibody is a single chain antibody.

6. The composition of claim 1, wherein said composition is recombinantly produced by fusing the gene encoding said antibody to the gene encoding gelonin.

7. The composition of claim 1, wherein said antibody is selected from the group consisting of TAb 250 and a humanized antibody bearing the variable region of Tab 250.

8. The composition of claim 1, wherein the antibody is an Fab or and F(ab')2.

9. The composition of claim 1, wherein the antibody is a humanized antibody.

10. A pharmaceutical composition comprising the composition of claim 1 and a pharmaceutically acceptable vehicle.